1. Field of the Invention
The present invention relates to a driving force distributing apparatus for a four wheel drive vehicle and, in particular, to a driving force distributing apparatus having a function of differential limiting by use of helical gears and an initial torque generating mechanism.
2. Prior Arts
The known driving force distributing apparatus for a four wheel drive vehicle includes a torque distributing apparatus (hereinafter referred to as a center differential apparatus) with a differential limiting function for properly distributing an input torque between front and rear wheels so as to improve a driving performance on a rough road or on a slippery road.
In order to produce a differential limiting torque, there have been proposed several types of differential limiting apparatuses and some of them has come into practical use.
As an example of disclosures, there is a differential proposed in Japanese patent application laid-open No. Toku-Kai-Hei 4-271926. The differential according to this disclosure includes a pair of side gears (worm gears) whose respective end is connected with drive axle ends respectively, a plurality of pairs of element gears (worm wheels) meshing with side gears and at the same time meshing with each other at respective gearing sections provided at both end thereof, a housing for accommodating and supporting the element gears, and bearing members such as thrust washers provided between the ends of the element gears and the housing, so as to generate a differential limiting torque proportional to an input torque.
Further, Japanese patent application laid-open No. Toku-Kai-Hei 5-280597 discloses a differential which comprises a first side gear connected to one output shaft, a second side gear connected to the other output shaft, a first pinion having a first gearing section at one end thereof which meshes with the first sun gear and having a second gearing section at the other end thereof, a second pinion having a third gearing section at one end thereof which meshes with the first gearing section and having a fourth gearing section at the other end thereof which meshes with the second gearing section and at the same time with the second side gear, and a housing having a plurality of holes for accommodating and rotatably supporting a pair of the first and second pinions therein, so as to produce a differential limiting torque by friction forces generated in the meshing portions of gears and the sliding portions between the pinions and the housing or between the pinions and the holes.
However, these torque proportioning type differential limiting apparatuses have such a problem that when the input torque is small, the differential limiting torque is also small. Because of this, in case where the vehicle enters into a road surface with a low friction coefficient or a state of one side wheel floating in the air while the vehicle is operated at relatively low load, the driving force enough for escaping from adverse road conditions can not be secured and the vehicle driving ability is deteriorated.
To solve such problem, it is considered that the when the differential revolution is initiated the differential limiting torque has been raised to some extent beforehand. This differential limiting torque is referred to as an initial torque hereinafter.
In case of the differential limiting apparatus using the combination of worm gears and worm wheels, as described in the aforementioned prior art, it is difficult to provide such a mechanism as to give an effective initial torque in a narrow space of the apparatus. Further, this type of the differential limiting apparatus requires a separate lubricating system because hypoid gear oil containing extreme-pressure additives is needed for lubricating gears. This is disadvantageous in employing automatic transmission oil (ATF) for both the automatic transmission and the differential limiting apparatus.
The case is similar to the differential limiting apparatus using the helical pinion gears and it is difficult to arrange required components so as to give an effective initial torque to the differential limiting apparatus.